It has been proposed to upgrade heavy hydrocarbons via hydroprocessing in which the hydrocarbons are admixed with an active catalyst composition in liquefied slurry form. As disclosed in U.S. Published Application No. 2007/0140927, the disclosure of which is incorporated herein by reference, a feed of heavy hydrocarbons and catalyst slurry is introduced into the lower portion of a reactor chamber, along with hydrogen in a gas phase. Those components travel upwardly within the chamber, enabling the hydrogen to react with, and hydrogenate, the hydrocarbons. Near an upper portion of the chamber, the hydrogenated hydrocarbons are removed as is excess hydrogen gas.
A flow of liquefied catalyst slurry and residual hydrogen gas is recirculated within the chamber through a vertically oriented downcomer in the chamber. Such a multi-phase mixture enters an upper end of the downcomer, for example, under the action of a recirculation pump. The mixing which occurs in the downcomer tends to keep the catalyst concentration profile and the temperature profile generally uniform along the height of the reactor.
It is desirable to separate gas from the multi-phase mixture descending through the downcomer, because the gas resists the downward flow of the mixture and reduces the efficiency of any pumps which can be provided to recirculate the mixture exiting the downcomer. Therefore, at or near its upper end the downcomer is typically provided with a degassing section, e.g., in the form of a generally frusto-conical pan which is upwardly open and leads downwardly to a usually cylindrical transport section of the downcomer. Due to the degassing section being of relatively large diameter, travel of the multi-phase mixture therein is slower than the natural ascension velocity of the hydrogen gas bubbles, thereby facilitating escape of the bubbles from the rest of the multi-phase flow.
That bubble escape reduces the overall resistance to downward flow of the multi-phase mixture, increases pump efficiency, and improves mixing among other advantages. It would be desirable to yet further reduce the bubble content in the multi-phase mixture.
In certain reactions, the deactivated catalyst becomes heavier, e.g., finely divided activated carbon used in a resid process. It would be desirable to further promote the separation of heavy (deactivated) catalyst from the unreacted catalyst before the mixture exits the downcomer, so that the heavy catalyst can be re-directed for regeneration.